As shown in FIG. 7, in the communication based on the FDD system, a base station transmits a DL signal with a frequency f(DL) in a DL frequency band which is defined in a predetermined operating band and a mobile terminal transmits a UL signal with a frequency f(UL) in a UL frequency band which is defined to be different from the DL frequency band.
Here, in the actual communication between the base station and the mobile terminal, a difference between the DL frequency f(DL) and the UL frequency f(UL) is defined for each operating band. A test device for testing the mobile terminal needs to check the operation of the mobile terminal at frequencies other than the frequency difference defined for each operating band. Therefore, the frequencies f(DL) and f(UL) can be arbitrarily set to at least the DL frequency band and the UL frequency band, respectively.
FIG. 8 shows the structure of a test device 10 for testing a mobile terminal 1 based on the FDD system. A DL radio-frequency signal RF(DL) with a frequency f(DL) which is output from a transmitting unit 11 is sent to the mobile terminal 1 to be tested through a coupler 12. A receiving unit 13 receives a UL radio-frequency signal RF(UL) with a frequency f(UL) which is output from the mobile terminal 1 through the coupler 12.
In the receiving unit 13, a mixer 13b mixes an input radio-frequency signal with a local signal L output from a local signal generator 13a. A low-pass filter 13c performs a high-frequency cutoff process on the output from the mixer 13b and performs heterodyne conversion to convert the input signal into a signal with a frequency in an intermediate frequency band which can be digitally processed. Then, an A/D converter 13d converts the converted signal IF(UL) into a digital signal string IFD(UL) and outputs the digital signal string IFD(UL) to a test processing unit 15.
The test processing unit 15 performs various kinds of signal processing including baseband conversion and data demodulation on the output from the receiving unit 13, generates a signal required for the test, for example, a signal IFD(DL) in the intermediate frequency band which is quadrature-modulated with a baseband signal, and sends the generated signal to the transmitting unit 11.
As described above, in the FDD system, since DL and UL are performed at the same time, some DL radio-frequency signals RF(DL)′ among the DL radio-frequency signals RF(DL) output from the transmitting unit 11 leak to the receiving unit 13.
Here, in the FDD mobile communication system, recently, there have been 21 operating bands which are defined in the range of about 700 MHz to 2700 MHz, as shown in FIG. 9. The base station and the mobile terminal transmit and receive signals in a UL frequency band and a DL frequency band of any of the operating bands (however, operating bands 15 to 18 are reserve bands).
The input band of the receiving unit 13 in the test device needs to be wide in order to respond to the many operating bands set in the wide frequency range. As described above, even when the directional coupler 12 is used between the transmitting unit 11 and the receiving unit 13, sufficient isolation is not obtained. The high-level DL leakage component RF(DL)′ and the UL radio-frequency signal RF(UL) are input to the receiving unit 13 at the same time.
In general, the receiving unit 13 of the test device 10 performs heterodyne conversion with a lower-side heterodyne system using a local signal L with a frequency f(L) that is lower than a reception frequency (band center frequency) by a predetermined frequency f(IF) (for example, 5 MHz) such that the frequency of the UL signal is converted into a frequency in the intermediate frequency band while the relationship between the frequency levels in the UL signal band is maintained.
Here, an example of operating band 8 in which the DL frequency band (925 MHz to 960 MHz) is higher than the UL frequency band (880 MHz to 915 MHz) will be described. As shown in FIG. 10(a), when a signal with an upper limit frequency f(UL) of 912.5 MHz in the UL frequency band is used as a UL signal with a frequency width of 5 MHz, the frequency f(L) of the local signal L is 907.5 MHz, which is 5 MHz lower than the upper limit frequency f(UL).
It is assumed that a DL signal with a frequency in the range of 5 MHz from a frequency f(DL) of 927.5 MHz which is closest to the frequency of the UL signal in the DL frequency band is transmitted.
When the UL signal RF(UL) with a frequency of 912.5 MHz and the DL leakage component RF(DL)′ with a frequency of 927.5 MHz are input to the receiving unit 13 and are subjected to lower-side heterodyne frequency conversion with the local signal L with a frequency f(L) of 907.5 MHz, the UL radio-frequency signal RF(UL) is converted into a signal IF(UL) with a frequency of 5 MHz and the DL leakage component RF(DL)′ is converted into a signal IF(DL) with a frequency of 20 MHz, as shown in FIG. 10(b).
Therefore, as shown in FIG. 10(b), when the cutoff frequency of the low-pass filter 13c is set to, for example, about 15 MHz, it is possible to remove the adverse effect of the DL leakage component on signal processing after the A/D conversion process.
The example of operating band 8 is an example in which the gap between the UL frequency band and the DL frequency band is the narrowest (10 MHz) which is the worst possible condition when the UL frequency band is lower than the DL frequency band. Therefore, when the UL frequency band is lower than the DL frequency band and both the UL and DL signal bands are about 5 MHz, lower-side heterodyne frequency conversion is performed for a UL reception frequency with a local signal with the frequency in the above-mentioned numerical example in order to remove the influence of the DL leakage component.
That is, in many operating bands in which the UL frequency band is set to be lower than the DL frequency band, when the lower-side heterodyne frequency conversion is performed using the above-mentioned frequency relationship, signal processing is hardly affected by the DL leakage component.
However, as in operating bands 13, 14, and 20 shown in FIG. 9, when the UL frequency band is higher than the DL frequency band and the lower-side heterodyne system is used similarly to the above, the DL leakage component enters the band of the low-pass filter 13c. 
For example, in operating band 20, as shown in FIG. 11(a), when a DL signal (center frequency: 818.5 MHz) having a frequency in the range of 5 MHz from the upper limit of the DL frequency band (791 MHz to 821 MHz) is used and a UL signal (center frequency: 834.5 MHz) having a frequency in the range of 5 MHz from the lower limit of the UL frequency band (832 MHz to 862 MHz) is used, the frequency f(L) of the local signal L is 829.5 MHz.
When heterodyne conversion is performed with the local signal L, the UL radio-frequency signal RF(UL) is converted into a signal IF(UL) with a frequency of 5 MHz and the DL leakage component RF(DL)′ is converted into a signal IF(DL) with a negative frequency (−11 MHz), as shown in FIG. 11(b). As the actual signal, the signal IF(DL) is symmetrically transferred with respect to a frequency of 0 and appears as a signal IF(DL)′ with a positive frequency of 11 MHz. The transferred signal IF(DL)′ enters the passband of the low-pass filter 13c. 
Here, when the DL leakage component RF(DL)′ has a high signal level, the A/D converter 13d is saturated or the dynamic range is narrowed by the leakage component even though the A/D converter 13d is not saturated. As a result, it is difficult to measure, for example, the power control of the UL signal.
Patent Document 1 discloses a technique for testing a mobile terminal in a different communication system, for example, a time division duplex (TDD) system. The technique controls the power of a transmission signal output from a test device to solve an interference problem in a structure in which a transmitting unit and a receiving unit are connected to each other through a coupler. However, in the technique, it is difficult to prevent the influence of the leakage of the DL signal which is output from the mobile terminal in the FDD system at the same time as the UL signal.